Photoconductive material for electrophotography



April 16, 1968 J. G. JARVIS 3,378,371

PHOTOCONDUCTIVE MATERIAL. FOR ELECTROPHOTOGRAPHY Filed April 8, 1965 AND TYPE A Z/IVC 0x105 //v RAT/0 90-40 RELATIVE POTENTIAL SATURATION POE/V7741.

I0 20 2; ADDITION JAMES 6 JARVIS N V EN TOR.

BYKZJW WWW ATTORNEYS United States Patent 3,378,371 PHOTOtZONDUCTIVE MATERIAL FOR ELECTROPHOTOGRAPHY James G. Jarvis, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Continuation-impart of application Ser. No. 19,040, Mar. 31, 1960. This application Apr. 8, 1965, Ser. No. 446,625

4 Claims. (Cl. 961.8)

This is a continuation-in-part of my application Ser. No. 19,040 filed Mar. 31, 1960, now abandoned.

This invention relates to electrophotography and particularly to photoconductive materials for use in xerography.

Xerography consists essentially of placing an electrostatic charge on the surface of a photoconductive layer, exposing the layer to an image to be reproduced so as to cause part of the charge to leak away and simultaneously or subsequently developing or toning the image by applying particles in the form of a powder or suspended in a liquid, to the differentially charged surface. In the simple forms of xerography, the particles are attracted to the areas which remain charged.

Various ways of charging the surface are known, one of the more satisfactory common methods involves passing the surface through an electric glow discharge or corona. When zinc oxide is used as the photoconductive material, the glow discharge has negative polarity, since zinc oxide retains a negative charge for a considerable time, unless and until illuminated. It is customary to charge the surface with a sufiiciently high intensity corona and for a sufficiently long time to reach what is termed saturation potential. This insures that the maximum amount of toner will be attracted to the unexposed areas and assures substantially uniform charge from one print to the next.

The exposure reduces the surface potential in propo tion to the amount of exposure up to the amount which completely removes the charge so that substantially no toner is attracted to the discharged areas. The absence of charge causes D (minimum density) in the corresponding areas of the ultimate print.

The quantity of exposure which reduces the surface potential to the value which barely gives D (maximum density) is the threshold exposure of the process. The higher exposure which effectively is just enough to remove all of the charge so as to give D,,,,,, is the greatest exposure which can be recorded by the process. The range between this threshold exposure and this greatest exposure which can be recorded, is referred to as the exposure scale. This refers to the scale over which density varies and hence it is the exposure scale for densities.

Certain reproduction processes, such as document copying, do not require full utilization of the exposure scale available. They are often operated with relatively short scale systems in order to provide high contrast. On the other hand, processes such as continuous tone reproduction and oscillograph recording (where the oscillograph spot sometimes moves slowly giving long exposure and other times moves fast giving short exposure) require a long exposure scale in the recording material. The present invention is particularly concerned with the latter group of processes, since its main object is to make possible the utilization of the longest scale available with any particular combination of recording material and toning process.

There is, of course, a direct and measurable relationship between surface potential, including saturation potential, quantity of charge on the surface, quantityof toner attracted to the surface and optical density of the print. Other factors are'of course, involved in these relationships.

The maximum charge on the surface is a function of the saturation potential, although the actual potential depends both on the quantity of charge and on other factors such as the dielectric constant. The support for the photocon- 'ductive layer is normally paper and this is, or is made,

and with the saturation potential of the recording layer.

The amount of exposure required to remove all of the charge from a given photoconductive layer depends on various factors, but is (or may be) more or less the same for all saturation potentials. There is a scale or range of exposures which may be identified, on the usual log E (log exposure) scale between some minimum value of exposure which produces no useful reduction of the charge and the maximum recordable value which removes substantially all of the charge. This exposure scale for charge removal (electric charge scale) is not necessarily, and is not usually, quite the same as the exposure scale for density (density scale) of the final reproduction, for reasons discussed in detail below. The difference is best explained in terms of an example.

For this example, assume two recording materials respectively having saturation potentials of 600 and 300 volts when treated by glow discharge, and, as is often the case, having identical or approximately the same electric charge exposure scales, say 1.0 unit on the log E scale. That is, 1.0 log E unit above that which barely has any effect on the charge, just removes all of the charge, other factors such as normal dark decay being allowed for. However, in this example, the particular toner process used subsequently to the exposure, is such that whenever the exposure leaves more than 250 volts surface potential, the optical density of the toner is a maximum, D,,,,,,, For one of the postulated materials, all areas retaining between this 250 volt value and the original 600 volt, will develop to D and for the other material, all areas having between 250 voltand 300 volt will develop to D,,,,,, For the 600 volt material, it requires about 0.5 or 0.6 log E unit to remove the first 350 volts (i.e. to reduce the potential to 250 volts). For this material, therefore, the exposure scale between D and D toner density (density exposure scale) is the remainder of the electric charge scale, namely 0.5 or 0.4 log E unit. The first 0.5 or 0.6 log E unit have no effect on the density.

For the 300 volt material on the other hand, only 0.2 or 0.3 log E unit exposure are required to lower the potential to 250 volt and thus 0.8 or 0.7 log E unit remain effective as density exposure scale. Thus the 300 volt material has a greater effective scale than the 600 volt material. With this particular toner process, a material having a saturation potential of exactly 250 volts would g ve he maximum exposure scale, namely one equal to the electric charge exposure scale. All of the electric charge exposure scale would be available as density exposure scale.

A material with still lower saturation potential, say volt, would also have a full exposure scale for density equal to the electric charge exposure scale, but the contrast would be much lower; the toner would never reach D,,,,,,, The resulting density would range from D (about zero) to some value below D but at the lower contrast the material would record all exposures within the electric charge exposure scale.

The object of the present invention is to provide a material which gives substantially Dmax, and still utilizes substantially all of the electric charge exposure scale.

Stated another way, it is the primary object of the 3 I I invention to provide a photoconductive layer of zinc oxide whose saturation potential is in the optimum range, namely the range which is high enough to give approximately maximum density in the unexposed areas of the material, but not substantially higher than the minimum value which will do just this. A range of useful values is discussed below.

There are two types of zinc oxide.

Both types have been well-known and commercially available since before the turn of the century. The terms type A and type F are here coined for the first time. These terms, by definition, mean the two well-known materials no matter how the materials are made. It is also well-known that the only really distinctive properties are the fact that the type here designated type A will allow negative charges to leak away and the type here designated type F will store such charges when in the dark.

Type F is sometimes referred to as French process or indirect process zinc oxide. This is because it is usually made by the burning of zinc after the zinc has been refined from zinc ore. Its identifying characteristic is the ability, when in a suitable vehicle or binder, to store electric charge. It may be referred to as charge-storing zinc oxide. This is a simplified term since it is not the zinc oxide crystals or particles by themselves which have this characteristic, but rather the zinc oxide particles in a binder, normally between and 50% by weight resin. As far as the present invention is concerned it does not matter how the type F zinc oxide or charge-storing zinc oxide is made. This product is well known and is identified herein as type F zinc oxide or as charge-storing zinc oxide, no matter how it is made.

Type A zinc oxide is sometimes referred to as American process zinc oxide or direct process zinc oxide, the latter term deriving from the fact that this zinc oxide is commonly made directly from a zinc ore by chemical treatment. Its distinguishing property is the fact that when in 10% to 50% by weight resin vehicle, it will not hold an electrostatic charge. This well known material is herein referred to as type A zinc oxide or as non-charge-storing zinc oxide.

Since the present invention involves a mixture of materials and in the preferred embodiment involves a mixture of the two types of zinc oxide, the definition of each type does not and must not extend to mixtures of the two. Of course any commercially available zinc oxide is made by only a single process whatever it is and to be absolutely rigorous in the definitions given here, type F by definition is a single process zinc oxide (e.g. the indirect process mentioned above) and type A by definition is also a single process zinc oxide (e.g. the direct-fromthe-ore process mentioned above). When the present invention comes into extended commercial use, mixtures of the two might conceivably become available but even this is most unlikely since the one preparing the sensitive sheets according to the present invention would seek optimum results and would mix the ingredients himself as described below.

The following is an objective, specific, test for identifying and distinguishing the two types of zinc oxide by their charge storing ability. It is purposely a critical test in which the layers being tested are made quite thin to create a maximum electric capacity and maximum field within the thin layer when the surface is charged to a given potential.

One starts with particles of some unidentified commercially pure single process zinc oxide. These particles routinely are between .08 and 0.8 micron mean particle size, although sometimes the particles are agglomerated and are ball milled to break up the agglomerations before the particles are dispersed in a vehicle. While specific percentages are given for the preparation of the tests samples, wide variations are permissible both in the materials and in the percentages. The results will be practically the same since type A zinc oxide has a low charge storing ability and type F zinc oxide has a high storing ability, and the difference shows up easily. The particles are dispersed in a resin in a ratio by weight of 60 units of Zinc oxide to 12 units of resin all in 125 units of solvent for the resin. In this simple test the resin is that in Pliolite S-7 described in Example 0 below and the solvent is xylene. Other resins and solvents can be used. The dispersion is coated on paper to attain a dry thickness between .0006 and .0012 inch and dried.

The sheetof paper is laid zinc oxide side up, on a grounded metal plate. Negative potential corona discharge from a horizontal thin Wire held at l0000 volts is applied to the front or upper surface of the zinc oxide layer by passing the sheet back and forth under the corona wire at a separation distance of about /2 to 1 inch, all in the dark or in a safe light except for the light of the corona itself, which is completely negligible. The charged sheet is then immediately placed under the probe of an electrometer spaced from the layer a distance of A to /z inch. Any electrometer can be used, for example that sold by General Radio Co. as Model 1230-A. When, as discussed below, the characteristic to be measured is the relative amount of charge at the beginning and after the passage of time, i.e., a ratio, several factors are not too critical. As indicated the thickness of the layer can vary over a range, the amount of charging is similarly not critical and the electrometer does not have to be calibrated in absolute values for each probe setting. The electric potential of the probe can be and usually is recorded as a curve of the potential vs. time. Alternatively the potential can be noted on a meter at the start of the test and then noted after a certain period of time as indicated by a stop watch or a watch with a second hand illuminated by a safe light, e.g., red light or by light not reaching the zinc oxide. Since this is to be a precise test, the atmospheric relative humidity is held at about 50% but of course there is no appreciable effect on the results if it is considerably drier or somewhat higher up to say The dark decay rate or decay factor is described in the same terms as leakage from a charged condenser, namely in terms of the time in seconds required for the potential V to fall to some predetermined fraction, usually l/e (namely 0.368) of its original value V V=V e where t is the time interval and K the decay factor. A decay factor of .01 means that the potential falls to .368V in seconds.

In a typical example the voltage V falls to .8V in 3 minutes seconds) which means that V would reach .368V in about 13 minutes (780 seconds) so that the decay factor is about .0013.

With this standard test, if the zinc oxide in resin layer shows a decay factor in the dark of the less than 0.01, it is, by definition, type F zinc oxide and is useful in the present invention as such. It is also useful in xerography generally. If on the other hand the layer shows a decay feature in the dark greater than 0.1, the zinc oxide is by definition type A zinc oxide and is not useful as a photoconductor in xerography but is particularly useful in the present invention as an additive. If any zinc oxide showed a decay factor between 0.01 and 0.1 (which seems most unlikely) it would be of no interest for the present invention either as a photoconductor or as an additive.

The various forms of vehicle, usually resin, in which the zinc oxide is incorporated for electrophotography are well known. These vehicles have a very high resistance which is reduced by the zinc oxide pigment, but is still very high compared to the paper support. The present invention does not involve novelty with respect to the support, the particular vehicle or the particular forms of type F zinc oxide which are used. Those commonly used are quite satisfactory for the present invention. The sensitivity of the type F zinc oxide can, as is well known, be varied in value and in spectral distribution by use of dyes. The

sensitivity may be affected adversely by the presence of foreign materials in the photoconductive layer.

The most pertinent factor to the present invention is the fact that type F zinc oxide of high sensitivity (in normal vehicles which do not adversely afiect the sensitivity) when coated as a layer on a conducting support, has a saturation potential produced by corona or glow discharge, which far exceeds the optimum saturation potential for normal toning procedures. As pointed out above, the object of the present invention is to reduce this saturation potential to the optimum value.

Attempts to reduce the charge of the unexposed layer either by controlling the intensity of the glow discharge and time of exposure to glow discharge or by a uniform flash pre-exposure to light introduced complications into the process and are not very satisfactory since the results are not uniform. Natural decay of surface potential with time (in the dark) operates to bring the potential into the optmium range, but this involves too much time delay for practical application. If materials which decay fast are employed, the time of decay is too critical and the results are not uniform.

According to the present invention on the other hand certain additives are included with the type F zinc oxide to form the pigment in the vehicle. That is, the pigment according to the present invention is a mixture of type F Zinc oxide and a small percentage of another pigment.

The additives must have certain properties. To be useful in the present invention, the additive must depress the saturation potential appreciably, preferably without reducing the ability of the layer to store charge. It happens that the materials which have proven successful for this purpose would, if used to replace all of the type F zinc oxide and to make up all of the pigment in the vehicle, cause the charge to leak away rapidly, but the additives are not used in such high percentages. Most additives which still permit storage of charge, do not depress the saturation potential and some actually raise the saturation potential and are hence not suitable to the present invention. The additive must not adversely affect the sensitivity of the zinc oxide. Some additives, in fact some of those which actually raise the saturation potential, reduce the sensitivity of the type F zinc oxide, and hence they are not suitable for the present invention.

According to the most preferred embodiments of the present invention, I have discovered four additives which, in proper percentages depending on the potential depressing factor (defined below) of the particular additive used, reduce the saturation potential of the layer by the amount necessary to bring this saturation potential within the optimum range of values discussed above, and at the same time these four additives do not adversely affect the sensitivity of the layer. The additives are: calcium oxide, lead carbonate, aluminum phosphate and type A zinc oxide.

The unique properties of these four additives are emphasized by the fact that if the following materials are added, the photographic speed or sensitivity of the photoconductive layer is decreased objectionably: titanium dioxide, magnesium oxide, aluminum silicate, calcium silicate, zirconium oxide and strontium sulfate. Some materials were tested but were found to be quite ineffective in reducing the saturation potential: silicon dioxide, barium carbonate, aluminum oxide, barium sulfate, calcium carbonate and zirconium oxide. Their potential depressing factor is less than .005 or is negative. In fact barium sulfate and calcium carbonate each caused the saturation potential to be higher than that of the type F zinc oxide alone in the vehicle.

The four preferred materials have different potential depressing factors. For example a very small amount of calcium oxide depresses the saturation potential as much as a much larger amount of lead carbonate. Also the potential depressing factor varies from one type A zinc oxide to another. However, the potential depressing factor, hereinafter referred to as D can be identified for any additive and is characteristic of that additive. D is defined by the equation:

D =log B-log E wherein D is the potential depressing factor, x is the percentage by weight of additive in the pigment, B is the saturation potential of the type F zinc oxide alone and E is the value to which this saturation potential is depressed by the additive. That is, E is the saturation potential of the layer in which the pigment is a mixture of type F zinc oxide anl x percentage by weight of additive. This equation represents a simple exponential curve. With straight type F zinc oxide, there is no additive; x equals zero. Hence E=B. With the 10% by Weight additive (x=10) E is a fraction of B depending on the effect of the additive, i.e. depending on the potential depressing factor D.

The value of x which must be selected to give the present invention is herein identified as P and this value P must be such that it reduces the saturation potential to a value giving a useful increase in exposure scale.

P must be such that it reduces the potential to .9 or .8 of its original value B (otherwise the effect of the additive is not appreciable). At the other extreme, P should never be greater than or by weight of the total pigment.

If M is the surface potential which just produces D the surface potential should, according to the invention, be reduced to a value between .75 and 1.33M. Thus the percentage by weight P of additive should be between l/D (log Blog 1.33M) l/D (log B log .75M). If 1.33M is greater than B, equal to B, greater than .913 or .8B, the corresponding value of P would respectively be less than zero, zero, less than .05/D or less than .097/D. Thus the value of P must also be greater than .05/D preferably greater than .097/D.

These mathematical equations are given to take into account the fact that the various additives have different potential depressing factors. Calcium oxide for example has a value of D about .066. For aluminum phosphate it is about .03. For lead carbonate it is about .01 and the various forms of type A zinc oxide which are available from various suppliers in this country, the value of D ranges from about .02 to about .08, generally .03 to .05.

A material with a potential depressing value D much less than .005 would have to be added in such large quantities that the sensitivity would be appreciably reduced. 20% of .005 material depresses the potential to .8B and even 50% merely reduces it to 56%. Materials with a value B greater than about .15 would depress the surface potential so effectively that the percentage of additive would have to be selected very precisely. Even 5% of such material depresses the potential to one tenth of its original value. Thus the materials must be selected in the general range of D values between .005 and .150. The four preferred materials mentioned above fall within this range.

The conflicting reports regarding the phosphorescence (as distinguished from fluorescence) of what is here called type F zinc oxide are noted. Some statements have been published to the effect that charge-storing zinc oxide (type F) is at least generally non-phosphorescent and that noncharge-storing zinc oxide (type A) is phosphorescent, the latter term being carefully defined with respect to the minimum period in microseconds which distinguishes phosphorescence from fluorescence. No reliance is here placed on phosphorescence as a useful distinguishing property since even if it were an appropriate criterion, it cannot be measured as precisely as decay factor.

The invention will be more fully understood from the examples given below and from the following description of the accompanying drawing in which:

FIG. 1 is a schematic flow chart representing a standard xerographic process.

FIG. 2 is a schematic section of a photoconductive material according to a preferred embodiment of the invention.

FIG. 3 is a graph to illustrate the effect of the invention on the exposure scale of the reproduction process.

FIG. 4 is a graph to illustrate the effect of additive according to the invention on the saturation potential of a photoconductive layer.

In FIG. 1 a photoconductive layer consisting essentially of type F zinc oxide in a resin on a conducting paper base 11 is charged by a glow discharge 12 created by an electrode 15 at high negative potential relative to a base plate 16, the potential being produced by a source indicated schematically at 17. The electrode 15 which is usually in the form of a fine wire parallel to the surface 10 is moved back and forth across the surface as indicated by the arrow 18.

The photoconductive layer 10 assumes a uniform negative electrostatic charge indicated at and for many minutes if the zinc oxide is kept in the dark, this charge 20 is maintained, decreasing to about 80% of its original value in about 3 minutes. A positive transparency 21 illuminated by a lamp 22 is imaged by a lens 23 on the photoconductive layer 10. Where the light strikes the photoconductive layer it causes it to become more conducting. In accordance with some theories of the operation of zinc oxide, the light breaks down an electric barrier layer or causes, in some way near the surface, a separation of electrons from holes (as the term is used in solid state physics) enabling the electrons, of relatively high mobility in zinc oxide, to move toward the base 11 while the holes with low mobility in zinc oxide, remain to neutralize the trapped or stored negative charges at 20. In any case, the effect is equivalent to that of increasing the conductivity of the layer 10 at the exposed areas allowing charges 20 to leak away to the base 11. This leaves an imagewise distributed electrostatic charge on the surface 10. This image is then developed or powdered in any of the known ways to deposit a toner 25 having optical density, in the unexposed areas of the surface 10. The print is a direct positive one relative to the transparency 21.

In order to assure uniformity of charge, it is customary to charge the surface 10 (either by the use of a highly concentrated glow discharge 12 or by repeated passing of the glow discharge electrode across the surface 10) to saturation potential. This insures not only uniformity over the surface, but also insures that the total charge is the same each time. As pointed out above, this charge is excessive. It must be adequate to produce D or substantially D for the image 25, but it must not be too much greater than this value since the exposure scale of the re production is cut short at higher values. An exposure scale lengthened by the present invention gives much better tonal quality in continuous tone reproduction.

A light uniform flash will reduce the surface charge, but this additional step is both inconvenient and difficult to reproduce accurately. Raising (if this were possible) or lowering the sensitivity of the zinc oxide would merely shift the response curve to lower or higher exposure value but would not appreciably change the scale. In any case, the highest sensitivity zinc oxide is, of course, preferable (other properties being equal). Mere waitin g until the dark decay rate brings the surface potential down to the desired value involves an impractical delay with all normal materials having a decay rate slow enough to be useful in Xcrography.

According to the present invention, the saturation potential and hence the quantity of the charge on the zinc oxide is matched to the toning process to give substantially maximum scale, by the addition of certain materials.

It has been found for example that the addition of certain materials reduces the saturation potential to a value between 20 and 80% of the original value. This degree of reduction brings the value within the range where D is still obtainable and an appreciably greater exposure scale is available for reproduction. That is, type F zinc oxide in resin has a saturation potential which is from 8 10 or 20% to 5 or more times too high for best image quality.

Furthermore the preferred additive materials are such that when added to the type F zinc oxide, the sensitivity of the layer to light is not appreciably changed. Certain other materials have been found to affect the sensitivity adversely.

And still further it is noted that the dark conductivity of the layer is not appreciably changed. When the layer is charged to saturation potential and allowed to stand for 3 minutes, the potential still just drops to about 80% of the original saturation potential. Certain other materials change the conductivity so that the charge leaks away too rapidly for the material to be useful.

Still other materials which do not affect the sensitivity of the zinc oxide and do not increase the dark decay rate (leakage) were found either to be ineffective, i.e. not to lower the saturation potential appreciably, or actually to raise the saturation potential. In general the increase in saturation potential was accompanied by a slight decrease in photosensitivity. It is apparent that there is no simple relationship between all of these properties.

FIG. 2 is a schematic enlarged section of the photoconductive layer 10 made up of type F zinc oxide particles 31, type A zinc oxide particles 32 all uniformly distributed in a binder 33 of resin. There is approximately 9 times as much type F zinc oxide as type A zinc oxide in this example.

In FIG. 3 the effect of exposure on the surface potential of a charged photoconductor is plotted for three different photoconductors. Curve 40 is for a material having a high saturation potential 41. Curve 42 is for a material having a medium high saturation potential 43 and curve 44 is for a material having a low saturation potential indicated at 45. As s customary in photoreproduction curves, the exposure is plotted on a logarithmic scale indicated as log E, where exposure E is intensity I times time t. Exposures having a value less than indicated at 50 are not sufficient to cause any appreciable drop in potential on the layer. These particular curves are corrected to eliminate the gradual drop in potential due to dark leakage. For all three materials an exposure represented by the value 51 is sufficient, for all practical purposes, to remove all of the electrostatic charge. Any residual charge beyond this is ineffective in the ultimate toning process. Thus, as far as electrical charge is concerned, all three of these materials have the same exposure scale, namely the value of log E between the points 50 and 51. Exposure less than 50 or greater than 51 have no appreciable effect on the potential.

However the reproduction process is such that a potential indicated by the horizontal broken line 55 represents sufficient charge on the photoconductor to collect enough toner to reach Dmax, This potential 55 is designated M. Higher values of the potential (on materials having curves 40 and 42) still just give the same Dmaxp Thus for material of curve 40, exposures less than the value 56 all give D and the only exposures which are reproduced as differences in density in the final print are exposures between the value 56 and the value 51. This is the reproduction scale of the material and process and obviously it is much less than the scale between the points 50 and 51.

When the saturation potential is reduced to 43 as for the material of curve 42, the exposure 57 at which the density begins to change from D is a much lower exposure value than the value 56. The effective range or scale of exposure between 57 and 51 is much greater than that between 56 and 51.

When the saturation potential is reduced exactly to match the D potential 55 of the toning process, the reproduction scale utilizes the total range between the points and 51. At still lower values (potential 45 is shown slightly lower than the value the full scale 9 between 50 and 51 is still utilized, but the D does not reach that obtainable with the particular toner process.

The amount of reduction in the relative potential which is still useful depends on how great an increase in reproduction scale is desired and how much less than absolute Dmax, can be tolerated in the final reproduction. If the range from 56 to 51 in exposure is inadequate, but the range from 57 to 51 is adequate, then it is necessary to use only sufficient material to lower the saturation potential to the value 43. Still better results could be obtained by lowering the saturation potential to the value 55 but useful results have already been attained in the curve 42. Similarly, while a maximum D is always desirable, useful results are obtainable with somewhat lower values and high quality continuous tone reproduction can be obtained even when the saturation potential is lowered to the value 45 somewhat below the optimum value 55. There is a range of useful saturation potentials (namely between and A, of the optimum potential 55) and there is a range with respect to the amount of materials which brings the saturation potential within his useful range. The limitations identified below represent values giving a high order of improved quality in the final reproduction.

In FIG. 4, a family of exponential curves is plotted to show the effect of adding different potential depressing agents. The curves are identified by their potential depressing factors D. In plotting these curves, the normal dark decay of charge with time, has been allowed for. Thus each curve represents saturation potential when the surface is first charged. In the absence of any additive, the type F zinc oxide in about 20% vehicle say, has a saturation potential indicated as 1.0 on the graph shown in FIG. 4. The addition of an additive to constitute 20% of the pigment, which aditive has a potential depressing factor D equal to .03 reduces the saturation potential to a value of .25 as indicated at the point marked 60 on the .03 curve. 20% of a different additive having a D value .02 reduces the saturation potential to 40% as shown at 61. Since additives which fail to reduce the saturation potential to .8 or .9 of the original value are of little value, percentages corresponding to the parts of the curves above the horizontal broken line 62 are of little interest and those above the line 63 are of no interest. It is for this reason that additives whose potential depressing factor is less than .005 are not normally employed.

The depression of the electrostatic saturation potential existing at time zero, which is defined above as depressing factor D, should not be confused with the decay factor K, which may or may not be changed appreciably by the additives. The decay factor involves the rate of change with time.

In FIG. 4 the saturation potential is drawn on a linear scale for clarity, but in practice a log scale may be used (semilog paper) which makes all of the curves straight lines and makes the range of .75M to 1.33M a constant increment on the (log) ordinate scale. In such a sernilog plot, it is easy to draw the line (curve) as soon as the value of the depressed potential is known for one percentage of additive. The intercept of this line with the horizontal line at .13 potential (10% of the original potential) gives l/D directly. This is also true for the curves in FIG. 4 but the lines on the semilog paper are easier to draw and interpret precisely than the curves n the linear graph (FIG. 4).

The following examples illustrate the operation of the invention:

EXAMPLE 0 (The control standard not employing the invention) A dispersion was prepared by mixing, in an electrically rotating blender for minutes, the following:

G. Pliolite S-7 (butadiene-styrene copolymer) 37.3 Silicone resin SR-82 (substituted polysiloxane) 4.7

G. Type F zinc oxide XX-78 56.0 Xylene 100.0 Methyl alcohol 2.0

The dispersion was coated on paper to attain a dry thickness of about 0.001 inch. Upon charging, this layer reached an initial potential of 340 volts which decayed to 82% of this value in 3 minutes, a decay factor of 0.0012.

Pliolite S7 is butadiene-styrene copolymer consisting of 45% butadiene, 55% styrene dissolved in xylene (30% resin 70% xylene by weight); it is supplied by Goodyear Tire and Rubber Company. Thus the above dispersion contains 11.2 g. butadiene-styrene and 126.1 g. xylene, the extra 26.1 g. being in the Pliolite S7. Silicone resin SR-82 is the product of a low degree polymerization of the hydrolyzate of mixed methyl and phenyl chlorosilanes; C H O Si with a phenyl content of about 40%; it is supplied by General Electric Co. Other polysiloxanes with substitute phenyl and methyl radicals on the silicon are available from various sources and, as is well known, they are satisfactory as binders for electrophotography. Zinc oxide XX-788 is a type F zinc oxide having a very high photosensitivity supplied by the New Jersey Zinc Co.

The quality of continuous tone prints made with this Example 0 is barely passable, although it is highly sensitive and quite useful in document copying.

I EXAMPLE 1 A dispersion similar to the control dispersion of Example 0 was prepared substituting insensitive type A zinc oxide (N o. 3038 McKesson and Robbins) for 10% by weight of the type F zinc oxide mentioned in Example 0. The initial potential obtained by saturation charging (by glow discharge) was volts (53% of the value obtained with Example 0) and this potential decays in the dark to 78% of its initial value in 3 minutes; a decay factor of .0014. The potential depressing factor D for this material is a little less than .03.

Much higher quality xerographic continuous tone prints are obtained with this material, the D being substan tially the same as that obtainable with the material of Example 0.

EXAMPLE 2 Example 2 was identical to Example 1 except that type A zinc oxide from a different source (XX503, New Jersey Zinc) was substituted. In this case the additive appeared to be slightly more effective in reducing the saturation potential but the dark decay rate was essentially unchanged and the quality of the picture obtained was essentially the same as for Example 1, although the D was a little lower. The saturation potential was about 28% of that obtainable with Example 0. Since 10% of the addition reduced the potential to 28%, the D value is a little higher than .05.

EXAMPLE 3 In Example -3 still another type A zinc oxide (Green Label No. 16, St. Joseph Lead Company) was tried, again substituting it for 10% by weight of the type F zinc oxide. Thus the Example 3 is essentially the same as Examples l and 2. In this case the initial saturation potential was 40% of the value for Example 0 and the dark decay rate was essentially unchanged. Thus the D is about .04.

EXAMPLE 4 In Example 4 type A zinc oxide (No. 3038) was substituted for 5% by weight of the type F oxide of Example 0. This reduced the saturation potential to about 70% of the Example 0 value, and gave an improved reproduction scale. Thus as in Example 1, No. 3038 type A zinc oxide has a D value a little less than .03.

1 1 EXAMPLE 5 In Example 5 type A zinc oxide of the type used in Example 1 (No. 3038 with a D value of a little less than .03) was substituted for 20% by weight of the type F zinc oxide of Example 0. This reduced the saturation potential to about 30% of the Example value and again in the decay rate was essentially unchanged.

In all of the above Examples 1 to 5, the sensitivity of the Zinc oxide is substantially unchanged, the D obtainable with the material is substantially unchanged, but the print quality and total scale are much better than with Example 0. In these examples type A zinc oxides with D values .03 to .05 are illustrated. Other type A zinc oxides having D values up to .076 have been used.

EXAMPLE 6 This is essentially the same as Example 0 except that aluminum phosphate was substituted for 20% by weight of the type F zinc oxide. This reduced the initial saturation potential to 35% of the Example 0 value and the dark decay rate was substantially unchanged. The D value is about .025. Again the sensitivity was also essentially unchanged and the quality of the resulting print was greatly improved. Since its D value is about .025, percentages of aluminum phosphate between 4% and 40% by weight of the type F zinc oxide give a useful improvement in exposure scale. 4% reduces the type F potential to .8 of its original value and 40% reduces it to .16 of its original value.

EXAMPLE 7 This was similar to Example 0 but lead carbonate was substituted for 20% by weight of the type F zinc oxide, which reduced the initial saturation potential to 64% of the value found for Example 0. Thus D equals .01. The dark decay rate and photosensitivity were essentially unchanged. The print quality was greatly improved. 10% to 50% by weight of the F type zinc oxide may be replaced by lead carbonate, according to the invention, to give useful results and to give saturation potential between .8 and .32 of its original value. When one reaches 40% or 50% by weight additive as the extremes in these Examples 6 and 7, the overall sensitivity starts to go down and hence greater amounts are not usually added. That is, when scale requirements call for greater depression of the saturation potential, it is preferable to use additives having a higher D value than aluminum phosphate and lead carbonate.

EXAMPLE 8 This is again similar to Example 0 but calcium oxide was substituted for by weight of the type F zinc oxide. This reduced the initial potential to 47% of the Example 0 value without adversely affecting the dark decay rate or the photosensitivity. Thus the D value is .066. Values between 1.5% and 5% of the type F zinc oxide may be replaced by calcium oxide with useful results giving potentials between .8 and .1 of the original value.

Mixtures of any of the mixtures also give useful results.

Replacing 20% by weight of the type F zinc oxide by barium sulfate was found to give a saturation potential about 10% higher than that for Example 0 and a decrease in photosensitivity was observed. Replacement of 20% by weight of the type F zinc oxide by calcium carbonate (which might be expected to be useful since calcium oxide and lead carbonate are among the useful materials) was also found to increase the saturation potential, in this case by Thus these materials have negative D values. The change in photosensitivity was only slight in this case, but was in the direction of decreased sensitivity.

With the various type A zinc oxides, D ranges from less than .03 to more than .05 (up to .076 for one material tested). Depression of potential to .8 requires from 3% (for .03) to 2% (for .05). 2% of the .03 material would depress the potential only to .87 but even this is useful to a degree. Similarly, at the other end of the scale, 20% of the .05 material leaves a potential of .1 of the original and 20% of the .03 material leaves still higher potentials of which values are useful to meet tonal and scale requirements. 20% of the .076 material reduces the potential to .03 of its original value which with most type F Zinc oxides and most toning processes, lowers the D to the limit of practical utility. Hence 2% to 20% by weight of the various type A zinc oxides are useful in the present invention.

As mentioned above titanium dioxide, magnesium oxide, aluminum silicate, calcium silcate, zrconium oxide, stron tium sulfate, silicon dioxide, barium carbonate, aluminum oxide, and zirconium oxide were also tried and found to be unsatisfactory for the present invention for one reason or another, although they may have other useful effects which are not part of or pertinent to the present invention. They either decreased the photographic speed or they were ineifective to reduce the saturation potential.

Thus the invention requires materials with D values between .005 and .15. To obtain appreciable effect (depression of the saturation potential to .8 (or .9) of the type F value B) the percentage P of additive must exceed .097/D (or .05/D) and so as not to interfere with the sensitivity of the type F zinc oxide, P must be less than 50% by weight of the pigment. Within these outside limits, P must be between 1/D (log B1og .75M) and 1/D (log B-log 1.33M)

to give an exposure scale which approximates the maximum available. The ideal value of P is:

l/D (log B-log M) but the .75M and 1.33 M values represent the useful range of approximations.

Having thus described several of the very successful examples of our invention, we wish to point out that it is not limited to the specific arrangements shown but is of the scope of the appended claims.

I claim:

1. A photoconductive sheet for use in xerography comprising a conducting support and on said support a photoconductive layer consisting of a vehicle, which relative to the support and to type F Zinc oxide, is insulating and distributed substantially uniformly through the vehicle, particles of inorganic compounds whose total weight is made up of to 98% by weight of particles of type F zinc oxide and 2% to 20% of particles of type A zinc oxide, whereby the electrostatic saturation potential of the sheet is less than that of a similar sheet with particles of type F zinc oxide only.

2. A photoconductive sheet for use in xerography comprising a conducting support and on said suport a photoconductive layer consisting of a vehicle, which relative to the support and to type F zinc oxide, is insulating, and distributed substantially uniformly through the vehicle, particles of inorganic compounds whose total weight is made up of particles of type F zinc oxide and particles of additive selected from the group consisting of 2% to 20% of said total weight, type A zinc oxide, 10% to 50% lead carbonate, 4% to 40% aluminum phosphate and 1.5% to 15% calcium oxide, whereby the electrostatic saturation potential of the sheet is less than that of a similar sheet with particles of type F zinc oxide only.

3. A photoconductive sheet for use in xerography comprising a conducting support and on said support a photoconductive layer consisting of a vehicle, which relative to the support and to type F zinc oxide, is insulating, and distriubted substantially uniformly through the vehicle, particles of inorganic compounds consisting of 0 to 100% of a mixture of type F zinc oxide particles and type A zinc oxide particles in the ratio by weight between 98:2 and 80:20, 0 to 100% of a mixture of type F zinc oxide particles and lead carbonate particles in the ratio by weight between 90:10 and 50:50, 0 to 100% of a mixture of type F zinc oxide particles and aluminum phosphate particles in the ratio by weight between 96:4 and 60:40 and 0 to 100% of a mixture of type F zinc oxide particles and calcium oxide particles in the ratio by weight between 98.5:l.5 and 85:15, the sum of the four mixtures being 100%, whereby the electrostatic saturation potential of the sheet is less than that of a similar sheet with particles of type F zinc oxide only.

4. A photoconductive sheet for use in a xerographic process in which the toner substantially reaches D at a value of surface potential of at least M, which sheet comprises a conducting support and on the support a layer containing a pigment and 10% to 50% by weight vehicle which alone has high resistance compared to the support and to the pigment, said pigment comprising a mixture of type F zinc oxide particles which alone in said vehicle would have a saturation potential B greater than M and P percent by weight of particles of an additive selected from the group consisting of calcium oxide, lead carbonate, aluminum phosphate and type A zinc oxide, said additive having a potential depressing factor D, defined by D =log B-log E where E is the saturation potential to which B is depressed by x percent of the additive, the value of P in said mixture being less than 5 0% and less than l/D (log B-log .75M)

greater than .097/D and also greater than l/D (log B-log 1.33M)

References Cited UNITED STATES PATENTS 2,937,944 5/1960 Van Dorn et al 96-1.8 2,979,402 4/1961 Greig 961.8 2,990,279 6/1961 Crumley et a1 96-1.8 3,003,870 10/ 1961 Jarvis et al. 96-1.7

C. E. VAN HORN, Examiner. 

1. A PHOTOCONDUCTIVE SHEET FOR SUSE IN XEROGRAPHY COMPRISING A CONDUCTING SUPPORT AND ON SAID SUPPORT A PHOTOCONDUCTIVE LAYER CONSISTING OF A VEHICLE, WHICH RELATIVE TO THE SUPPORT AND TO TYPE F ZINC OXIDE, IS INSULATING AND DISTRIBUTED SUBSTANTIALLY UNIFORMLY THROUGH THE VEHICLE, PARTICLES OF INORGANIC COMPOUNDS WHOSE TOTAL WEIGHT IS MADE UP OF 80% TO 98% BY WEIGHT OF PARTICLES OF TYPE F ZINC OXIDE AND 2% TO 20% OF PARTICLES OF TYPE A ZINC OXIDE, WHEREBY THE ELECTROSTATIC SATURATION POTENTIAL OF THE SHEET IS LESS THAN THAT OF A SIMILAR SHEET WITH PARTICLES OF TYPE F ZINC OXIDE ONLY. 